1. Field of the Invention
The present invention relates to a shape and a structure of a magneto-resistive element and is applied to a magnetic random access memory, in particular.
2. Description of the Related Art
Conventionally, a variety of types of solid magnetic memories have been proposed. Recently, a magnetic random access memory using magneto-resistive elements that exhibit a giant magneto-resistive effect is proposed, most of which magneto-resistive elements utilize a ferromagnetic tunnel junction.
A ferromagnetic tunnel junction has a laminated structure of, for example, ferromagnetic layer/insulation layer (tunnel barrier layer)/ferromagnetic layer. A tunnel current flows through the insulation layer, when a voltage is applied to the insulation layer in this structure. In this case, tunnel conductance of the ferromagnetic tunnel junction changes in proportion to a cosine of a relative angle between the two ferromagnetic layers.
That is, when the two ferromagnetic layers are magnetized in the same direction (parallel to each other), the junction resistance value is minimized, and when they are magnetized in the opposite directions (anti-parallel to each other), the value is maximized.
Such a phenomenon is called a tunneling magneto-resistive (TMR) effect. For example, recently, a rate of change (MR rate) in resistance value of a magnetic tunnel junction (MTJ) element owing to the TMR effect is reported to be as high as 200% or more.
In a magneto-resistive element having a ferromagnetic tunnel junction, one of two ferromagnetic layers is defined as a pinned layer whose magnetized state is fixed and the other is defined as a free layer whose magnetized state changes with data. For example, if the pinned layer and the free layer are magnetized parallel to each other, stored data is supposed to be “0”, and if they are magnetized anti-parallel to each other, it is supposed to be “1”.
Data is written by, for example, applying a magnetic field generated by a write current flowing through a write line to a magneto-resistive element to thereby reverse a direction of magnetization of the free layer of the magneto-resistive element. On the other hand, data is read by, for example, supplying a read current through a ferromagnetic tunnel junction of the magneto-resistive element to thereby detecting a change in resistance of the ferromagnetic tunnel junction owing to the TMR effect.
By arranging such magneto-resistive elements as memory cells in an array, a memory cell array is configured. A variety of types of structures have been proposed for the memory cell array. For example, a one-transistor/one-MTJ type structure is known in which one switching transistor is connected to one magneto-resistive element in order to enable fast random access.
Another structure is proposed in which a diode and a ferromagnetic tunnel junction are connected in series in a region where a word line and a bit line intersect with each other.
However, to realize a magnetic random access memory practically, a lot of problems are to be solved.
For example, a disturbance problem must be solved which is caused by dispersion of a switching magnetic field of magneto-resistive elements that constitute a memory cell array. Further, it is also necessary to manuplate an astroid shape so that the magneto-resistive elements have write selectivity (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-273337, U.S. Pat. No. 6,005,800, by Y. K. Ha et al., 2004 Symp, VLSI Technol. Dig. Tech. Papers, p. 24).
Further, it is also necessary to enable both fine patterning of magneto-resistive elements and reduction in a write current. Presently, if the magneto-resistive element is made smaller, its coercive force becomes larger, so that a larger switching magnetic field is required to reverse a magnetization direction. In this case, it is difficult to reduce a write current required to generate a magnetic field, thereby increasing power dissipation.
In addition, it is necessary to improve thermal stability to store data for a prolonged lapse of time. Thermal stability, which is judged by a parameter called a energy barrier ratio, α=ΔE/kBT, is preferably larger. However, the energy barrier ratio is proportional to a volume and coercive force of a ferromagnetic layer, so that it is deteriorated as the magneto-resistive elements are fine-patterned more.